In the transmission of data between a first Data Terminal Equipment (DTE) and a second DTE over telephone lines, each DTE is equipped with a modem wherein a carrier signal is modulated by the data in a modulator at the transmitting end and demodulated in a demodulator at the receiving end.
In the today modems, the data bit stream received from the DTE at the input of the modem is loaded in a serializer/deserializer which provides parallel groups of bits, the number of bits in each group depending on the data bit rate of the modem. Each group is converted to a point in a phase-amplitude diagram, all the points of the diagram forming a constellation. Each point is then translated into a couple of values corresponding to the coordinates of the point. The technique for coding the points of the constellation is described in the article entitled "Multidimensional Signal Constellations for Voice-band Data Transmission" by A. Gersho and V. Lawrence, published in IEEE JOurnal of Selected Area in Communications, vol. SAC-2, Nov. 5, 1984.
Then, these two quadrature signal values are modulated by a carrier signal before being spectrally shaped in a filter centered at the carrier frequency providing as an output a number of samples of the shaped signal each baud samples are provided to a digital-to-analog converter in order to be converted into an analog signal to be sent over the telephone line.
Reciprocally, on the other direction, the analog signal received from the telephone line is first converted into digital samples. The samples are filtered, and the output of the filter representing two in-phase and quadrature components, are used to provide a point in the plane corresponding to a group of bits. The juxtaposed groups of bits are then serially transmitted to the DTE.
Whatever be the type of telephone lines used, leased lines or public lines of the switched telephone network, the carrier signal is often affected by a frequency shift. Indeed, the signal can be frequency switched one or more times in the equipments of the telephone network. As these equipments are not sufficiently interdependent, the signal which is received by the modem receiver is no longer maintained at the carrier frequency of the transmitting modem. As a consequence, such a frequency shift, which can be of about from 0,1 Hz to 6 Hz, results in a phase variation at each baud time which has to be taken into account by a phase filter incorporated in the modem receiver.
The phase filters, used in the today modems such as the IBM modem 5866 are of the second-order loop type wherein the frequency shift is first computed by accumulating the phase error evaluated every baud time, and then the phase correction to be added to the modem equalizer output to compensate for the frequency shift is computed at each baud time by an updating of the phase correction of the preceding baud obtained from a combination of the current value of the frequency shift and the current value of the phase error. Such phase filters enable the frequency shift and the resulting phase correction to be computed with an accuracy sufficient for transmission rates not greater than 14 400 bits/s. But, with a rate as high as 19 200 bits/s, the accuracy provided by the existing phase filters does not enable problems raised by phase disturbances to be solved.
Such phase disturbances are principally due to line breaks and phase hits. A line break consists in a drop of energy of the signal received by the modem. During a line break, the modem switches to a mode of operation where it expects a synchronization signal. Nothing allows the modem to distinguish between a line break and a normal end of transmission. It is only when the energy is detected at the receiver input within the modem receive range that the modem can know that it was a line break. In such a case, it is necessary that the value of the phase correction be identical to what it would have been in case no energy drop occurred. Such a condition can be reached only if the value of the frequency shift which has been saved just before the line break is sufficiently accurate. If not, the error in the value of the frequency shift leads to an error in the value of the phase correction which rapidly exceeds a value of some degrees (5 degrees for a 99 200 bits/s modem) for which the modem is no longer able to get proper decoding decisions.
Another phase disturbance occurs in case of a phase hit which happens on the telephone line. If such a phase hit is higher than a value of some degrees (always 5 degrees for a 19 200 bits/s modem), the value of the phase correction at the output of the phase filter is suddenly off the correct setting due to the value of the phase hit. The modem becomes unable to provide significant errors that would drive the phase filter towards the correct phase correction. The decoded decisions are random and many high amplitude errors occur, which result in a high mean-square error.